Hermetic batteries such as primary and secondary batteries are assembled by arranging battery elements such as a positive electrode, a negative electrode and an electrolyte in a cylindrical battery case having a closed end and sealing an opening of the battery case with a lid and a gasket.
If foreign matter such as a piece of metal comes into the battery case in the assembly process and internal short circuit occurs, output voltage of the battery may be decreased or unfavorable gas may be generated. Therefore, the entry of the foreign matter in the assembly process should be prevented for improved battery reliability.
The battery case is formed by drawing a metallic plate thin to form a cylindrical body having a closed end and cutting part of the cylindrical body at an opening thereof along its circumference. In general, the cutting is carried out by pressing using a punch and a die. However, the pressing inevitably leaves burrs on the cut surface of the cylindrical body because a certain clearance lies between the punch and the die. The occurrence of the burrs may be reduced by optimizing the clearance, but the burrs cannot be completely eliminated.
If there are burrs on the cut surface of the cylindrical body or an end face of the battery case, problems may arise when the battery elements are placed in the battery case. More specifically, an active material mixture layer on the surface of a wound positive or negative electrode may partially be peeled off when it comes into contact with the burrs. If a peeled piece of the layer enters the battery case, it may cause internal short circuit. The burrs generated on the end face of the battery case may also come off to cause external short circuit between the battery case and the lid (sealing plate), thereby causing voltage drop. Further, if the gasket comes into contact with the burrs when it is fitted in the opening of the battery case, the gasket may partially be scratched, which may possibly decrease the hermeticity of the battery.
In order to solve these problems, it is necessary to prevent the generation of the burrs as much as possible in the step of cutting the closed-end cylindrical body. However, since the burrs are inevitably generated, it is more practical to take measures to prevent the entry of the foreign matter in the battery case.
As an example of such measures, Patent Literature 1 discloses a method for forming a closed-end cylindrical battery case provided with a reverse-tapered opening having a larger diameter than the diameter of a middle part thereof. Since the opening is reverse-tapered, burrs formed on the end face are prevented from coming into contact with the gasket for sealing the battery case. Further, when the battery elements are placed in the battery case, contact between the active material mixture layer and the burrs is also avoided. Accordingly, the entry of the foreign matter in the battery case is prevented.
As another example, Patent Literatures 2 and 3 disclose a method for curving the end face of the reverse-tapered opening of the battery case. This method makes it possible to prevent the opening from scratching the gasket and the foreign matter such as a peeled piece of the active material mixture layer from entering the battery case.
The end face of the opening is curved in the following manner. As shown in FIG. 7B, a closed-end cylindrical body 100 is formed to have an opening having a reverse-tapered part 101 having a larger inner diameter d2 than the inner diameter d1 of the middle part of the cylindrical body 100 and another reverse-tapered part 102 having a larger inner diameter d3 than d2. Then, the reverse-tapered part 102 is cut off in the direction parallel to the side face of the cylindrical body 100 (in the direction of an arrow A depicted in FIG. 7B) to leave a rounded inner face at the opening of the battery case as shown in FIG. 7A.
Patent Literature 1: Japanese Unexamined Patent Publication No. 2001-52656
Patent Literature 2: Japanese Unexamined Patent Publication No. 9-161736
Patent Literature 3: Japanese Unexamined Patent Publication No. 10-321198